1. Field of the Invention
This invention relates to a method for treating a subterranean reservoir penetrated by a well, and more particularly concerns a method for more uniformly acidizing relatively high temperature subterranean reservoirs penetrated by a well.
2. Description of the Prior Art
Acidizing of wells is a well-known process for increasing or restoring the permeability of subterranean reservoirs to thereby facilitate the flow of reservoir fluids, such as oil, gas or geothermal fluid, into the well from the reservoir, and also to facilitate the injection of fluids through the well into the reservoir. Acidization involves treating the reservoir with an acid in order to dissolve the reservoir rock itself or clogging deposits, such as carbonate scale and siliceous deposits, thereby opening pores and other flow channels and increasing the permeability of the reservoir.
Numerous acidization methods have been proposed to cope with varying well conditions and special reservoir problems. However, in recent years the increased activity in drilling very deep oil and gas wells and geothermal wells has outpaced the development of suitable acidization methods, primarily due to the high temperature of these reservoirs.
A problem common to many prior art acidization processes is the corrosion of the well equipment, particularly the downhole tubing and casing, which is exposed to the acidizing fluid. Because the reactivity of an acid is significantly increased at higher temperatures, the corrosion of well equipment is especially serious in the acidization of high temperature reservoirs.
Corrosion inhibitors are generally incorporated into the acidizing fluid prior to its injection into the well. However, the effectiveness of the known corrosion inhibitors decreases at higher temperatures, and the expense of the corrosion inhibitors, which are significant even at low temperatures, become prohibitive at temperatures above about 250.degree. F. Another difficulty with the known corrosion inhibitors, especially when used in the large quantities required in high temperature wells, is their tendency to form insoluble solids upon reaction with reservoir materials, thereby damaging the reservoir.
Another problem in the acidization of high temperature reservoirs is that the acid is rapidly consumed by the reactive material immediately adjacent the borehole before the acid can penetrate any significant distance into the reservoir. Without adequate reservoir penetrations, the acidization operation is of little value. In view of these problems, many prior art acidization methods are limited, as a practical matter, to the acidization of reservoirs having temperatures on the order of 250.degree. F. and less.
The aforementioned problems have been overcome to a large extent by the use of methods disclosed in my U.S. Pat. Nos. 4,180,360, 4,203,492, and 4,267,887 wherein a substantially anhydrous acid precursor is injected into the reservoir and is allowed to hydrolyze in situ to generate a hydrohalic acid. In the last of the aforementioned patents there is described a well treatment, especially suitable for treating hot dry gas wells which do not contain water, wherein there is injected down an injection tubing a treating fluid, comprising a foam of an acid precursor, a gas and a foaming agent, while water is injected either along with the treating fluid down the same injection tubing as a plurality of small, discrete slugs or simultaneously down the injection tubing-casing annulus. The treating fluid and the water mix in the borehole and the resulting mixture then passes out into the reservoir.
There are several objectives in using acid precursors to treat high temperature reservoirs. First, the acid precursor should not hydrolyze to form an acid to any great extent during its passage through the well equipment in order to avoid corrosion of the well equipment. Second, the acid precursor should begin to hydrolyze as soon as possible after it enters the reservoir so as to increase the permeability of the reservoir in the critical area in the immediate vicinity of the borehole. Third, the acid precursor should not completely hydrolyze until the treating fluid has been injected a substantial radial distance out into the reservoir so as to increase reservoir permeability a substantial distance away from the borehole. Fourth, in order to accelerate the rate of hydrolysis these two immiscible phases which have different densities, i.e., acid precursor and water, should be intimately mixed together so as to maximize their interfacial area of contact and minimize their gravity segregation.
While the above-described methods for acidization utilizing acid precursors have proven quite useful in treating many high temperature reservoirs, there exists a need for a further improved acidization treatment, especially such a treatment wherein the permeability in the immediate vicinity of the borehole is substantially improved.
Accordingly, it is a primary object of this invention to provide a method for acidizing high temperature subterranean reservoirs utilizing acid precursors.
It is another object to provide such a method wherein the permeability of the reservoir in the immediate vicinity of the borehole is improved.
It is a further object to provide such a method utilizing as a treating fluid an intimate mixture of an acid precursor and water.
It is a still further object to provide such a method wherein the interfacial area of contact between the acid precursor and the water is increased.
It is yet another object of the invention to provide such a method wherein the rate of hydrolysis of the acid precursor is increased by decreasing the particle size of the acid precursor in a treating fluid.
Other objects, advantages and features of the invention will become apparent to those skilled in the art from the following description and appended claims.